1. Field of the Invention
The present invention relates to a picture image reader in facsimile, for example, which reads the picture image information in a color original by irradiating the original to be transmitted and by receiving the reflected or transmitted light from the oringal.
2. Description of the Prior Art
In facsimile and others, for reading picture images of an original to be transmitted, use is generally made of a solid sensor array consisting of CCD sensor or the like with wide reading width that can read a plurality of lines simultaneously. With such a device, there has been known a reading method in which, as shown in FIG. 1, the picture image information of the original 10 corresponding to the reading width of the sensor array is scanned electronically or mechanically in a first direction, namely, the main scanning direction, in one reading operation in the main scanning direction. Then, the original is moved by a predetermined distance, namely, by the amount of a pitch that corresponds to the reading width of the sensor array, in the subscanning direction which is perpendicular to the main scanning direction, to carry out the next reading operation in the main scanning direction. The reading operation of the original for the device consists of a repetition of the scanning operation as described in the above.
Now, for reading picture images in the main scanning direction by means of a sensor array, there are known also several other methods. Namely, a method in which use is made of a reducing optical system whose lens is utilized for illuminating the picture image reading area of the original, as well as for focusing the picture images corresponding to the full length in the main scanning direction by reducing the length, in order to let the sensor array receive the reflected light from the picture image in the original. Another is an erecting, unit-magnification, and contact imaging method which supplies picture image information to the sensor array via a rod lens array which is arranged to the full width in the main scanning direction, with one end of the array facing the reading position of the picture image of the original and the other end facing the sensor array. Still another is a series reading method which reads the picture image information by scanning the original in series successively in the main scanning direction by moving the sensor array in the main scanning direction. Here, the method of utilizing a reducing optical system and the method of erect, unit-magnification, and contact imaging type are the methods of scanning the original electrorically in the main scanning direction. However, there are drawbacks such as, in the method of utilizing a reducing optical system, the structure of the device becomes bulky due to increase in the optical length for the lens, and in the method of erect, unit-magnification, and contact imaging type, it becomes costly due to the necessity of using a plurality of CCD sensors that correspond to the width in the main scanning direction. In contrast, the series reading method is a method in which scanning in the main scanning direction is carried out mechanically, so that it has an advantage that the device can be made compact and economical.
Hence, in constructing a color facsimile unit, it is desirable from the viewpoint of economy and miniaturization to adopt the series reading method which reads the picture image information in the original in succession by scanning the original in series along the main scanning direction while moving the sensor array in the main scanning direction.
However, the series reading method has a drawback in that the operation of reading by scanning takes too long a time because it reads the picture images by scanning the original in succession in continuous manner.
Moreover, in reading the original by scanning the original in the main scanning direction, in one reading scan it reads the picture images in the region that corresponds to the reading width of the sensor array, and in a subsequent similar reading scan in the main scanning direction it reads the picture images in the region that corresponds to the reading width of the sensor array. In this way, in each reading scan it reads the picture images that are located in the region corresponding to the reading width of the sensor array. When each of the picture image information with a width of region that is read in succession, is arranged in regular order along the subscanning direction, it forms a reproduction of the original picture images in the original. However, there is a problem that the picture image information read by the sensor that is on one edge section of the sensor array differs from that read by the sensor that is on the other edge section, even if the same picture image is read by both of them. This is due to the difference in characteristics of sensors on both edge sections of a sensor array with fixed reading width, difference in illumination and difference in precision of mechanism at both edge sections, such as the difference in the distances from each of the sensor on both edge sections to the original and the difference in shading corrections. Because of this, if each picture image information read is simply arranged in regular order in the direction of subscanning, there is generated a discontinuity at each boundary of picture images, namely, at the boundary of each main scanning. It then leads to a result that the picture image is distorted unnaturally due to the discontinuity. In particular, in the case of color picture images, there are generated not only variable density in the picture image but also a difference in the hue, so that the distortion becomes more conspicuous.
Further, there is another method, as shown in FIG. 2, in which picture images are read by a plurality of line sensors 14 and 16 placed separated in the direction of main scanning. In this method, the portion 19 of the picture elements, which is the overlap between the two sensors, is processed in which the two output signals for picture images that are output from the sensors 14 and 16 are switched electrically to form one line of continuous output signal for picture images, with an arbitrary picture element in the overlapped picture elements as boundary.
However, according to the system, there are output picture image signals that are different with the juncture as the boundary due to scatter in the electrical characteristics (namely, nonlinear characteristics, offset, and so forth) of each line of sensors, which produces a juncture in the reproduced picture images.
Namely, human vision has a special feature that it has a very high ability of detecting the relative difference between two images that are displayed for comparison so that if the juncture of the output signals from the two line image sensors becomes noticeable, then the image is perceived as a band-like unevenness with width that corresponds to the length of the sensors in the main scanning direction, which is an unsightly image.
In particular, human eyes are more sensitive to the difference in hue than to the difference in brightness. Then, in a color image reader, if there is a scatter in the electrical characteristics in the line image sensors or a scatter in the spectroscopic characteristics of color filters that are placed in front of each of line image sensor, then the juncture between the output signals from the line image sensors becomes very conspicuous. Since it is difficult to suppress these scatters to a low level by selection or combination alone of image sensors or color filters, there is a problem that the burden on the price is high also.